A pattern formed on an object under check such as a mask, printed substrate, etc., is scanned by a CCD camera, and check pattern data for one scanning line obtained from the CCD camera is divided into a plurality of pixel groups and stored in shift register groups. The check pattern data is compared with a master pattern data representing the external shape of a master pattern to calculate an inclination of the check pattern (pattern to be checked) with respect to the master pattern. The order of reading addresses of the shift register groups is changed in accordance with the inclination so that the check pattern data matches the master pattern data. Thereafter, the check pattern data is compared with the master pattern data to detect a defect in the check pattern to thereby avoid erroneous judgment due to an angular deviation between the check pattern and the master pattern.